Casing for a marking instrument



FIG. 1 is a bottom, left front perspective view of a casing for a marking instrument showing my new design;

FIG. 2 is a left side elevational view;

FIG. 3 is a right side elevational view;

FIG. 4 is a top plan view;

FIG. 5 is a bottom plan view;

FIG. 6 is a front elevational view; and,

FIG. 7 is a rear elevational view thereof. 

The ornamental design for a casing for a marking instrument, as shown and described. 